Electrical contact assembly having damping means

ABSTRACT

An electrical contact for a conductive shaft such as a shaft for a moving coil in an electrical meter, the contact comprising a housing containing a mobile conductive medium such as mercury, which connects the shaft to an input or output conductor. The mobile conductive medium is normally retained within the housing by virtue of a small running clearance for the shaft. The contact is vibration proof within a range of acceleration forces and vibration frequencies Damping of the shaft may be effected by a liquid mobile conductive medium.

O Umted States Patent [151 3,674,968 Langdon 1 July 4, 1972 [54]ELECTRICAL CONTACT ASSEMBLY 2,840,782 6/1958 Ammon ..'..33s/222 x HAVINGDAMPING MEANS 3,406,339 10/1968 Bibbings ..335/222 x [72] Inventor:Thomas Edward Lllllgdon, 10 Park g X Crescent Elstree England can l x3,478,265 11/1969 Sauser ..335/222 x [22] Filed: July 20, 1970 PrimaryExaminer-H. 0. Jones [21 1 Appl' 56287 Attorney-Baldwin, Wight & Brown[30] Foreign Application Priority Data [57] ABSTRACT y 25, 1969 GreatBritain An electrical contact for a conductive shaft such as a shaft fora moving coil in an electrical meter, the contact comprising a U-S- Cl-H, hou ing containing a mobile conductive medium such as mer. [5 hilt.Clcury hi h connects th h ft t an i p t or tp t d [58] Field Of Search..200/166 H, 166 C, 335/47, 54, on The mobile conductive medium isnormally retained 335/222- 193 within the housing by virtue of a smallrunning clearance for 56 R i cud the shaft. The contact is vibrationproof within a range of ac- 1 e I celeration forces and vibrationfrequencies Damping of the UNITED STATES PATENTS shaft may be effectedby a liquid mobile conductive medium. 786,696 4/1905 Vreeland ..335/222x 12 Claims, 3 Drawing Figures ELECTRICAL CONTACT ASSEMBLY HAVINGDAMPING MEANS This invention relates to an electrical contact for rotaryconductive shafts and more particularly to such a contact adapted foruse with electrical meters.

In, for example, moving coil ammeters it is usual practice to mount acoil wound bobbin between shafts running in jewelled bearings.Electrical contact is made with each end of the coil through hairsprings located about respective shafts, one end of each spring beingattached to its respective shaft and the other end being rigidly mountedand firmly attached to a conductor supplying current to the coil via thespring. In such an arrangement each hair spring allows limited rotarymovement of the coil and at the same time conducts current fordisplacing the coil against the return action of the spring. In anotherknown construction taut bands are used in place of the hair springs.These known arrangements have the disadvantage that when used in anexplosive atmosphere, should one of the current carrying hair springs ortaut bands break leaving separated ends, relative movement of these endsmay be sufficient to cause a spark to ignite the explosive atmosphere.Safety precautions may be taken by enclosing the moving coil ammeter inan explosionproof or flameproof housing but this leads to further costand in some cases it is functionally undesirable.

One of the aims of the present invention is to provide an electricalcontact particularly suitable for making an electrical connection withthe shaft of, for example, a moving coil in an electrical meter, whichcontact is not subject to an explosion hazard and at the same time doesnot restrict the movement of the shaft in its bearing.

In accordance with the broadest aspect of this invention, an electricalcontact for a conductive shalt, said shaft being movable relative saidcontact, comprises a housing arranged to accept the shaft or adjacentthe shaft, the location of the shaft in or relative to the housing beingsuch that a mobile conductive medium contained in the housing isretained between the shaft and the housing without said medium normallybeing able to escape from the housing, said medium making electricalcontact either between the housing which is conductive and the shaft, orbetween an electrical conductor attached to the housing adjacent themobile conductive medium and the shaft.

The mobile medium making contact between the shaft and a stationaryconductor is preferably mercury, but it may be another conductive liquidor granulated conductive solid both of which are capable of maintainingelectrical contact with the shaft and of not escaping from the housing.Suitable media other than mercury may be liquid carbon suspension,conductive greases or granulated conductive solids not liable to loseelectrical contact with the rotary shaft.

in a preferred embodiment the electrical contact of this inventionprovides an explosion-proof or flame-proof electrical contact for anelectrical meter, the shaft being one which is attached mechanically andelectrically to a moving coil of the electrical meter. The shaft ismounted in a bearing within the housing and passes through an aperturein the housing. The space between the shaft and the housing is filledwith mercury so as to make electrical contact between the conductivehousing or the electrical conductor attached to the housing and theshaft. The clearance between the shaft and the aperture in the housingis such that the mercury is normally unable to escape from the housingdue, it is thought, to various factors, including surface tensionalforces and capillary action.

in the preferred embodiment the housing may be a conductive body havinga central bore containing a jewelled bearing or bearings. The rotaryshaft passes into the bore through the aperture in the body and issupported on the bearing(s), a chamber or chambers being formed betweenthe shaft and the interior walls of the bore. Under normal circumstancesof mechanical shock or vibration the mercury is retained within thechamber(s).

When the electrical contact is used in a temperature varyingenvironment, allowances must be made for expansion of the mobile mediumsuch that it is free to expand without escaping from the clearance.

Exemplary embodiments will now be described with reference to theaccompanying drawings in which:

FIG. 1 illustrates a side view of a moving coil meter and shows one ofthe bearing arrangements in side section,

FIG. 2 illustrates a side sectional view of a bearing housing containingpart of a shalt, and

FIG. 3 is an end-on view of the housing shown in FIG. 2.

Refen'ing to FIG. 1 which illustrates a side view of a moving coilmeter, a coil wound bobbin 1 is fixed to shafts 2 and 3 respectivelysupported on bearings 4 (only one of which, i.e. that supporting shaft2, is shown). Bearings 4 are located in conductive housings 5 and 6,respective bearings accommodating the ends of the shafts 2 and 3.Bearings 4 may be of known type, for example, jewel bearings. Housings 5and 6 are supported, for example, by screw threads on the housings whichengage tapped holes in fixed frame members 7 and 8. The ends of eachhousing 5 and 6 are slotted to accept a screwdriver blade. This allowsadjustment of the distance between the housings and hence the bearings 4to provide the correct end float on shafts 2 and 3.

A biasing member, such as a coil or hair spring is attached to the shafi2, or 3, or to both, and fixed relative the shalt, to provide returnbias when the coil wound bobbin 1 is displaced by an input current.

Considering the housings 5 and 6, but referring to housing 5 which issectioned in FIG. 1 for the purpose of illustration and explanation, anaperture 11 allows the shaft 2 to pass into the housing 5 for locationon its bearing 4. The bearing 4 is preferably a jewel bearing whichtherefore insulates the shaft from the conductive housing 5, the housing5 being insulated from the sides of shaft 2 because of its clearance,i.e. the diameter of aperture 11 is greater than that of the shaft sothat the shaft does not touch the sides of the aperture. Shafts 2 and 3are electrically connected to leads l2 and 13 of a coil wound on thebobbin 1. Electrical contact is established between the housing 5 andshaft 2 by filling a chamber 14 formed between the shaft 2 and the innersurfaces of housing 5 with a conductive mobile medium such as mercury.Such a medium allows electrical contact to be made between the housing 5and the coil lead 12, a similar contact being made between housing 6 andcoil lead 13. Thus, if the housings 5 and 6 are electrically connectedto input ten'ninals to the moving coil meter, the moving coil may besupplied with input current for measurement. The mercury allowselectrical contact to be maintained with the rotary shaft 2 (or 3) whileallowing relatively free movement of the shaft 2 (or 3) in its bearing4.

The clearance between the aperture walls adjacent the shaft 2 (or 3)must be such as to prevent mechanical friction therebetween andsimultaneously prevent escape of the conductive mobile medium, e.g.mercury. Such a clearance may be determined by practice and is thoughtto be ascertainable by considering the properties of the conductivemobile medium such as its surface tension, and to the properties of theparticular arrangement related to capillary forces in the clearance andpartial vacuums in the housing 5 (or 6).

The shafi 2 may be stepped, or provided with a collar 9, an end face 10of which is opposite and spaced from the corresponding end face ofhousing 5. The annular spacing between the collar 9 and the end face 10is such as to assist in preventing escape of the mobile conductivemedium (e.g. merc y), should any of the medium escape or tend to escapefrom the shaft clearance at aperture 11. A suitable shaft clearance fora shaft 2 of 0.015 in. diameter is 0.0075 in., the diameter of aperture11 being 0.030 in. A suitable spacing between collar 9 and end face 10is 0.025 in.

The preferred conductive mobile medium is mercury, but other mediums areenvisaged such as liquid carbon suspensions, conductive greases, orgranulated conductive solids not liable to lose electrical contact withthe rotary shaft 2 (or 3) or escape from the clearance at the aperturein the housing 5 (or 6).

FIGS. 2 and 3 illustrate an alternative bearing arrangement in which aconductive housing contains bearings l6, l7 and 18 spaced apart byspacers 19 and 20. Bearing 18 is an end bearing for the rounded end of ashaft 21, and bearings 16 and 17 are sleeves supporting the shaft 21.All of the bearings are preferably jewel bearings. One or more sleeves16 and 17 may be provided, two being preferable.

Chambers 22 and 23 are formed between the housing and shaft 21, thesechambers being filled with the conductive mobile medium, for example,mercury. Electrical contact is thereby made between the conductivehousing and the shaft 21 and hence to a moving coil for example mountedon the shaft 21, and an input lead of the electrical meter containingthe bearing arrangement of FIG. 2. The clearance between the sleevebearings 16 and 17 (particularly bearing 16) allows ro tary movement ofthe shaft without allowing the conductive mobile medium to escape fromthe housing 15. An aperture 24 through the end of the housing 15 andthrough a spacer 25 allows unrestricted rotary movement of the shafi.

FIG. 3 is an end-on view of the bearing arrangement'shown in FIG. 2, andillustrates a slotted end 26 to accept a screwdriver for adjusting theposition of housing 15 by threaded movement in a tapped supporting frameas previously described.

The use of a liquid mobile conductive medium (e.g. mercury) may alsoprovide a form of damping for the shaft and in particular for a metermovement. Viscous drag will occur on for example, the shaft 2 of FIG. 1,the drag being increased by providing ridges, fins or otherprotuberances on the shaft 2.

The invention is not restricted to containing the conductive mobilemedium in a bearing housing as explained above, as it is also possibleto arrange a sleeve about the shaft supporting a moving coil of anelectrical meter, the sleeve having a chamber containing the medium andhaving such a clearance about the shaft to prevent its escape from thechamber. Such sleeves could be fitted to the shafts of known metermovements in place of current carrying hair springs.

In a further modification, a container filled with the conductive mobilemedium may be mounted adjacent a meter shaft and with such clearancetherefrom as to prevent the escape of the medium from the container butallowing the medium to contact the shaft.

Referring to the drawings, it is not necessary for the housings 5 and 6of FIG. 1, or 15 of FIG. 2 to be wholly conductive, as a conductor couldbe set in an insulating housing provided it contacted the conductivemobile medium and an electrical input for the shaft.

The principal advantage of the embodiments described above, particularlywith respect to their use in electrical meters used in explosiveatmospheres, is that stressed current carrying ligaments are dispensedwith such as hair or coil springs which are liable to fracture undernormal use. Electrical meters fitted with the electrical contact of thisinvention do not suffer from the hazard of a spark occurring betweenfractured ends of a current carrying hair or coil spring.

In order to demonstrate the utility of the invention, vibration testswere carried out on a meter movement fitted with an electrical contactas illustrated in FIG. 1 herein and including mercury as the mobileconductive medium. Such a movement was made and tested to the DefenceSpecification DEF-62, Sealed Electrical Indicating Instruments" whichspecifies a range of vibration tests varying from 0.5 g acceleration ata vibration frequency of 10 c.p.s. to 5.8 g at c.p.s. Even with the coilaxis of the meter movement mounted vertically, the meter movementsatisfactorily passed the tests. In fact only on exceeding the maximumtest requirements, namely by using an acceleration of 6 g at a frequencyof 750 c.p.s. did mercury emerge from the electrical contact.

What we claim is:

1. An explosion resistant electrical contact assembly coml'ififsiisifiififii'i ififif ffiiiffi f'ffife iififi means including mountingmeans defining a chamber surrounding a portion of said shaft, saidchamber having an opening through which said shaft extends, a mobileconductive medium within said chamber in direct electrical contact withsaid shaft, the relative positions of said shaft and said chamber beingsuch that nomially said mobile conductive medium would flow through saidopening, and said opening being a restricted opening defined by a wallportion spaced from said shalt a distance sufficiently great to preventcontact between said shaft and said wall portion and sufliciently smallto retain said mobile conductive medium.

2. An electrical contact assembly according to claim 1 in which thechamber is cylindrical and said wall edges form a lip defining saidopening which encircles a cylindrical outer sur face of said shaft, thespacing between the lip and said cylindrical outer surface being suchthat said mobile conductive medium is normally unable to escapetherethrough.

3. An electrical contact according to claim 2 in which said spacing issubstantially 0i0075 inches and said mobile conductive medium ismercury.

4. An electrical contact according to claim 3 in which the lip is anannular ridge of rectangular section with an inner surface circularlyparallel to said cylindrical outer surface of said shaft.

5. An electrical contact according to claim I in which said mobileconductive medium is mercury.

6. An electrical contact assembly according to claim 1 in which saidmobile conductive medium is a conductive grease not liable to loseelectrical contact with said shaft.

7. An electrical contact according to claim 1 in which said mobileconductive medium is a liquid carbon suspension not liable to loseelectrical contact with said shaft.

8. An electrical contact according to claim 1 in which said mobileconductive medium is a granulated conductive solid not liable to loseelectrical contact with said shaft.

9. The electrical contact assembly of claim 1 wherein the other of saidbearing means includes similar mounting means.

10. The electrical contact assembly of claim 1 wherein each of saidbearing means is an end thrust bearing, and said shaft is supportedsolely by said end thrust bearings.

11. The electrical contact assembly of claim 1 wherein at least said onebearing means includes an end thrust bearing and a sleeve bearing, andsaid sleeve bearing defines said restricted opening.

12. The electrical contact assembly of claim 1 wherein said mountingmeans is in the form of a closed end tubular member having externalmeans for adjustably mounting the same.

1. An explosion resistant electrical contact assembly comprising aconductive shaft, bearing means supporting opposite ends of said shaftfor rotation, at least one of said bearing means including mountingmeans defining a chamber surrounding a portion of said shaft, saidchamber having an opening through which said shaft extends, a mobileconductive medium within said chamber in direct electrical contact withsaid shaft, the relative positions of said shaft and said chamber beingsuch that normally said mobile conductive medium would flow through saidopening, and said opening being a restricted opening defined by a wallportion spaced from said shaft a distance sufficiently great to preventcontact between said shaft and said wall portion and sufficiently smallto retain said mobile conductive medium.
 2. An electrical contactassembly according to claim 1 in which the chamber is cylindrical andsaid wall edges form a lip defining said opening which encircles acylindrical outer surface of said shaft, the spacing between the lip andsaid cylindrical outer surface being such that said mobile conductivemedium is normally unable to escape therethrough.
 3. An electricalcontact according to claim 2 in which said spacing is substantially0.0075 inches and said mobile conductive medium is mercury.
 4. Anelectrical contact according to claim 3 in which the lip is an annularridge of rectangular section with an inner surface circularly parallelto said cylindrical outer surface of said shaft.
 5. An electricalcontact according to claim 1 in which said mobile conductive medium ismercury.
 6. An electrical contact assembly according to claim 1 in whichsaid mobile conductive medium is a conductive grease not liable to loseelectrical contact with said shaft.
 7. An electrical contact accordingto claim 1 in which said mobile conductive medium is a liquid carbonsuspension not liable to lose electrical contact with said shaft.
 8. Anelectrical contact according to claim 1 in which said mobile conductivemedium is a granulated conductive solid not liable to lose electricalcontact with said shaft.
 9. The electrical contact assembly of claim 1wherein the other of said bearing means includes similar mounting means.10. The electrical contact assembly of claim 1 wherein each of saidbearing means is an end thrust bearing, and said shaft is supportedsolely by said end thrust bearings.
 11. The electrical coNtact assemblyof claim 1 wherein at least said one bearing means includes an endthrust bearing and a sleeve bearing, and said sleeve bearing definessaid restricted opening.
 12. The electrical contact assembly of claim 1wherein said mounting means is in the form of a closed end tubularmember having external means for adjustably mounting the same.